tty: serial: exar: generalize RS485 setup

Move the non-board-specific part of the RS485 initialization from
iot2040_rs485_config function to a new generic function used also for
other boards.
This allows using TIOCGRS485 and TIOCSRS485 on boards (such as mPCIe
serial IO modules) which are hard-wired to RS485 or have jumpers for
their configurations.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>